1. Field of the Invention
The present invention relates to an optical waveguide element which can be used as a second-harmonic-generation (SHG) device of quasi-phase-matched system (QPM), a method for producing the same and a method for producing a periodical domain-inverted structure.
2. Related Art Statement
As a beam light source of a blue laser beam light used for optical pick up etc., a SHG device of QPM system using an optical waveguide element having a periodic domain-inverted structure on lithium niobate or lithium tantalate single crystal has been expected. Such a device can be used to a broad range of applications including optical disc memory, medical use, photochemical use, and various optical measurements. For example, Japanese Patent Application Laid-open No. 4-104,233 describes that a film is grown on a ferroelectric optical single crystal substrate which was subjected to a treatment of producing single crystal domains (to be referred as "single-domained", hereinafter) by a liquid phase epitaxial growing process, and using the film having Curie temperature higher than the film-forming temperature (liquid phase epitaxial temperature) at that time, thereby to produce a ferroelectric optical single crystal film which is single-domained opposite to the substrate. The film is considered to become multi-domained when film has Curie temperature lower than the film forming temperature.
However, such a method could produce only a film of a composition having Curie temperature higher than the liquid phase epitaxial growing temperature and hence the composition of the film is very narrowly restricted, so that the method could not be used for practical uses. Particularly, when a domain-inverted structure produced by this method was used for a SHG device etc., damage caused by the beam light was liable to increase and hence the proportion of the output to the input of SHG became small, so that a practical device could not be provided.
As a process of providing a protruded and recessed structure for QPM on a surface or a main surface of the ferroelectric substrate, a dry process like a reactive ion etching process has been used. However, the method gave large damages to the substrate to largely deteriorate the crystalline property of the substrate. Thus, when the method was used for producing a SHG device, the conversion efficiency and the resistance to optical damage of the SHG device were decreased.
Because the conversion efficiency of a SHG device is improved in proportion to the power density of the fundamental wave, an increasement of the waveguide power of the of the fundamental wave by means of an optical waveguide is effective. For that purpose, the optical waveguide has to be changed to a channel waveguide structure of a three dimensional structure. However, in order to provide a channel waveguide, an ion exchange, metal diffusion or dry etching process like a reactive ion etching process is necessary which affects a large damage to the substrate to deteriorate the crystalline property of the substrate. Thus, the resistance to optical damage and conversion efficiency of the optical element of the SHG device were decreased. In addition, the producing process became complicated to decrease the production efficiency.